古骨中可溶性、不可溶性胶原蛋白的氨基酸组成和C、N稳定同位素比较分析

COMPARATIVE ANALYSES OF AMINO ACIDS AND C,N STABLE ISOTOPES BETWEEN SOLUBLE COLLAGEN AND INSOLUBLE COLLAGEN WITHIN ARCHAEOLOGICAL BONES

提取考古遗址出土的人和动物骨中的不可溶性胶原蛋白(ISC),进行C、N稳定同位素分析,是目前古食谱分析的主要研究方法。然而,对于无法提取出ISC的骨样,古食谱的重建也就无从谈起。之前,我们通过凝胶层析法,首次成功提取出古骨中ISC的降解产物——可溶性胶原蛋白(SC),并发现:SC与ISC的C、N稳定同位素比值相似,同样可以应用于古食谱研究。为进一步探明两者稳定同位素相似的机理,本文选取更多的古骨样品,开展了SC和ISC的元素含量、稳定同位素值和氨基酸组成分析。研究结果表明,SC与ISC的总氨基酸组成,完全符合I型胶原蛋白的特点。此外,SC与ISC相比,8种必需氨基酸组成,非常接近,变异范围仅为0.25%(n=3);而11种半必需氨基酸和非必需氨基酸,则变化稍大(0.90%,n=3)。显然,不同性质氨基酸含量的少许差异,是造成两种蛋白C、N稳定同位素相似但不相同的主要原因。

The carbon and nitrogen stable isotopic ratios of bone collagen at archaeological sites can reflect human and animal diets directly, helping us understand their food resources and living environment. Generally, the protocol to extract the collagen mainly focuses on the gelatinized collagen extracted from bones, i.e., the insoluble collagen （ ISC ） , as it widely believed that the ISC is the most resisted to diagenesis. However, due to bad bone preservation, it is natural that enough ISC cannot be extracted sometimes. Our preliminary study（ Wang et al.,Science in China： Earth Science, in press） showed that the degraded products of the ISC within acid solution which was separated through gel chromatography, i.e. soluble collagen （SC） , can still maintain the similar C, N stable isotope values to those from ISC, suggesting that SC has the great potential for palaeodietary reconstruction as well. But, the reasons and the mechanism on why ISC and SC had similar isotopic values are not understood well. Therefore, this present study try to explain the reasons and give the reasonable interpretations based on the comparative analyses of amino acids and stable isotopes between ISC and SC. Four bone samples, including one dog, one domestic pig and two cattle, were chosen from the Xiaoshuangqiao site（XSQS） ,located in Xiaoshuangqiao village（34°51＇N, 113°33＇E）, about 20km to the northwest of Zhengzhou City,Henan Province. Basically,the XSQS was dated to the Middle Shang period （ about 3400aB. P.） according to analysis of the archaeological remains. ISC was extracted according to the traditional protocol, which the collagen was got after gelatinization. SC was separated from the acid-soluble solution through gel （Cellufine GCL-2000） chromatography system, which was described in details in our previous study （Wang et al., Science in China：Earth Science, in press）. The elemental contents, stable isotope values of C and N as well as the amino acids from SC and ISC were measured by IRMS and LC-MS/MS for comparison. Results showed that the amino acid composition in SC and ISC is quite similar, containing high contents of Gly, Pro and 5-Hyp, which is the typical of characteristics of the type I collagen. This means that the two proteins extracted from bones were from the same collagen although they had different C and N contents. The difference for δ13C mean value is only about 0.2‰ （ n = 3 ） and 0.6‰ （ n = 3 ） for 815N value, confirming again that SC can be regarded as the potential material for palaeodietary study. It is notable for us to observe that the essential amino acid concentrations（EAAs） in SC and ISC are quite close. The mean difference of EAAs is only 0.25% （ n = 3）. Compared with the above, the mean difference （ 0.90%, n = 3） of semi-essential amino acids （SEEAs） and non- essential amino acids（NEEAs） between ISC and SC is a little larger. It is well known that there is little isotopic fractionation from dietary protein, especially the EAAs, to collagen. Therefore, we believe that the relatively stability of EAAs in SC can account for the similarity between SC and ISC,which makes the SC as another potential subject for palaeodietary reconstruction when the ISC was absent.